[Dogma21cw]

I don't know if you're still paying attention to this thread. But I have done some research for something I am working on and have some more information for you.

fetch_musername is the function & hook that creates the html marked up username. This is what allows you to colour names based on usergroup and such.

Now, near as I can tell, to achieve what you desire you will need to do the following:

1. Create a user profile field to contain your display username.
2. Create a template and register the variable for dusername.
3. Query the sql table for the field you created in step 1 and assign that string to dusername.
4. NOT RECOMMENDED You can now overwrite the musername function through a plugin substituting the dusername variable for the username one currently in the function. This is simplest, but use it at your own risk.
5. The other method is to create your own function fetch_dusername, copy the musername function, subbing as I suggested above, setting it up as a hook as well. Then going through your templates and replacing all references to musername and fetch_musername.

It's a bit of a PITA the second way. And probably someone better at me with php and vBulletin will be able to offer you better direction. But since no one spoke up, I thought I'd share the information I had.